The term "thermal transfer printing" covers two main areas of technology. In thermal transfer printing of textiles, a donor sheet is coated with a pattern of one or more dyes, contacted with the fabric to be printed, and heat is uniformly administered, sometimes with concomitant application of a vacuum. The transfer process has been much studied and it is generally accepted that the dyes are transferred by sublimation in the vapor phase. See, e.g., C. J. Bent et al., J. Soc. Dyers Colour., 85, 606 (1969); J. Griffiths and F. Jones, ibid., 93, 176, (1977); J. Aihara et al., Am. Dyest. Rep., 64, 46, (February, 1975); and C. E. Vellins in "The Chemistry of Synthetic Dyes", K. Venkataraman, ed., Vol. VIII, 191, Academic Press, New York, 1978.
The other area covered by the term thermal transfer printing is thermal imaging where heat is applied in an imagewise fashion to a donor sheet in contact with a suitable receptor sheet to form a colored image on the receptor. In one type of thermal imaging, termed thermal mass transfer printing as described, for example, in U.S. Pat. No. 3,898,086, the donor is a colorant dispersed in a wax-containing coating. On the application of heat, a donor layer in the construction melts or is softened, and a portion of the colored donor coating transfers to the receptor. Despite problems with transparency, pigments are generally the colorants of choice to provide sufficient light fastness of the colored image on the receptor. Another type of thermal printing is termed thermal dye transfer imaging or recording or dye diffusion thermal transfer. There, the donor sheet contains a dye in a binder. On imagewise application of heat, the dye, but not the binder, is transferred to the receptor sheet. A recent review has described the transfer mechanism as a "melt state" diffusion process quite distinct from the sublimation attending textile printing. See P. Gregory, Chem. Brit., 25, 47 (1989).
This same review emphasizes the great difficulty of finding or synthesizing dyes suitable for diffusive thermal transfer, stating that "It is significant that of the one million or so dyes available in the world, none were fully satisfactory." Among the failings of the dyes are inadequate light and heat fastness of the image and insufficient solubility of dyes for coating in the donor sheet. As has been noted previously, light fastness is also a problem in mass transfer imaging systems. In fact, achieving adequate light fastness is probably the single biggest challenge in these constructions. In large measure this is the result of the diffusive thermal transfer dye image being a surface coating a few microns thick. The dye is thus readily susceptible to photooxidative degradation. In contrast, textile fibers, which are 100 times thicker, are uniformly dyed throughout their depth, so that fading in the first few microns at the surface is of little practical importance. Consequently, it is common to find that dyes showing good light fastness in textile printing exhibit very poor photostability in diffusive thermal transfer imaging (see, e.g., U.S. Pat. No. 4,808,568) and thus, there remains a strong need for improved dyes for the latter application.
The thermal printing art in teaching the use and production of full color images [Mitsubishi Kasei R & D Review, 3, (2), 71-80 (1989)] states that "in order to achieve a recorded good showing wide color reproduction range, it is necessary that the absorption spectral characteristics of the three primary color dyes be correct." It is noted that "each dye should absorb one-third of the visible wavelength band while allowing the remaining two-thirds to be transmitted, and show high color purity, which does not allow overlapping of each absorption." Additionally, the art (i.e., U.S. Pat. No. 4,923,846) teaches that ". . . in heat transfer recording, if the color characteristics of the three colors of cyan, magenta, and yellow are not [low], the intermediate colors become turbid colors with low chroma, whereby no good color reproducibility can be obtained."
Although thermal printing of textiles bears a superficial resemblance to diffusive thermal dye imaging, they are in reality quite different processes with distinct properties and material requirements involved. Thermal printing occurs by a sublimation process, so that substantial vapor pressure is a prime criterion for dye selection. In diffusive dye imaging, high vapor pressure of the dye contributes to undesirable thermal fugacity of the image. For the melt state diffusion process involved in this situation, melting point is instead a better basis for dye selection. Diffusive dye transfer is a high resolution dry imaging process in which dye from a uniform donor sheet is transferred in an imagewise fashion by differential heating to a very smooth receptor, using heated areas typically of 0.0001 square inches or less. In contrast, the thermal printing of textiles is of comparatively low resolution, involving contemporaneous transfer by uniform heating of dye from a patterned, shaped or masked donor sheet over areas of tens of square feet. The typical receptors printed in this manner are woven or knitted fabrics and carpets. The distinct transfer mechanism allows such rough substrates to be used, while diffusive thermal dye imaging, where receptors with a mean surface roughness of less than 10 microns are used, is unsuitable for these materials.
Unlike diffusive thermal dye imaging, the transfer printing process is not always a dry process; some fabrics or dyes require pre-swelling of the receptor with a solvent or a steam post-treatment for dye fixation. Though the transfer temperatures for the two processes can be similar (180.degree. to 220.degree. C.), diffusive dye transfer generally operates at somewhat higher temperatures. However, in a manner strikingly reflective of the differences in mechanism involved, diffusive dye transfer involves times of around 5 msec, whereas thermal printing normally requires times of 15 to 60 sec. In accord with the sublimation process involved, thermal printing often benefits from reduced atmospheric pressure or from flow of heated gas through the donor sheet. Thermal printing is a technology developed for coloring of textiles and is used to apply uniformly colored areas of a predetermined pattern to rough substrates. In contradistinction, diffusive dye transfer is a technology intended for high quality imaging, typically from electronic sources. Here, a broad color gamut is built with multiple images from donors of the three primary colors onto a smooth receptor. The different transfer mechanism allows the requirement for gray scale capability to be fulfilled, since the amount of dye transferred is proportional to the heat energy applied. In thermal printing, gray scale capability is expressly shunned because sensitivity of transfer to temperature decreases process latitude and dyeing reproducibility.
U.S. Pat. Nos. 5,223,476 and 5,304,528 disclose dyes for thermal printing that have two vinyl aniline moieties joined by a linking group, each vinyl aniline moiety containing two "electron withdrawing groups" at the terminus of the double bond. While .beta.-cyano-.beta.-ethylsulfonyl- and .beta.-cyano-.beta.-arylsulfonyl-p-dialkylaminostyrenes are used in examples, no disclosure is made of .beta.-cyano-.beta.-trifluoromethanesulfonyl-p-aminostyrenes. Japanese Patent Publication No. JP 2-292371 describes similar styryl dyes for thermal printing, the .beta.-cyano-.beta.-ethylsulfonyl group again being exemplified. Japanese Patent Publication No. 3-086591 describes magenta dyes for thermal printing that comprise the .alpha.,.beta.-dicyano-.beta.-sulfonamide moiety. Neither Japanese patent mentions any potential advantage or utility of .beta.-cyano-.beta.-trifluoromethanesulfonyl-containing aminostyryl dyes. Furthermore, while the bis(trifluoromethanesulfonyl)-p-dialkyaminostyryl dyes are disclosed for use in thermal printing as eutectic mixtures with other dyes in U.S. Pat. No. 4,857,503, and the .beta.,.beta.-dicyano-p-dialkylaminostyryl dyes are disclosed for use in thermal transfer printing in a number of patents, including U.S. Pat. Nos. 4,833,123; 4,701,439; and 4,999,026, there is no mention in any of these documents that the "mixed" version of the dyes, i.e., the .beta.-cyano-.beta.-trifluoromethanesulfonyl-p-dialkylaminostyrenes, may have improved light fastness, lower hue error, and lower turbidity as thermal printing donor element dyes.